Prostaglandin synthesis

ABSTRACT

PROSTAGLANDINS ARE SYTHESIZED FROM O-METHOXYPHENYLACETIC ACID BY INTRODUCTION OF A PROPER ACID SIDE CHAIN, CONVERSION OF THE AROMATIC NUCLEUS TO THE CYCLOPENTANE NUCLEUS, AND INTRODUCTION OF THE SECOND SIDE CHAIN. NOVEL COMPOUNDS HAVING A HIGH LEVEL OF BIOLOGICAL ACTIVITY ARE DISCLOSED.

3,644,502 PROSTAGLANDIN SYNTHESIS Robert B. Morin, Douglas 0. Spry, andKenneth L.

Hauser, Indianapolis, and Richard A. Mueller, Bloomington, Iud.,assignors to Eli Lilly and Company, Indianapolis, Ind. No Drawing. FiledDec. 16, 1968, Ser. No. 784,225 Int. Cl. C07c 61/36 US. Cl. 260-514 R 1Claim ABSTRACT OF THE DISCLOSURE Prostaglandins are synthesized fromo-methoxyphenylacetic acid by introduction of a proper acid side chain,conversion of the aromatic nucleus to the cyclopentane nucleus, andintroduction of the second side chain. Novel compounds having a highlevel of biological activity are disclosed.

BACKGROUND OF THE INVENTION The prostaglandins are members of a newhormonal system, the discovery of which dates back to 1930 whenphysiological effects which were later found to be ascribable toprostaglandins were reported. The members of this family are Cunsaturated acids containing a five-membered ring in the structure. Anumber of naturally 'occurring prostaglandins have been reported todate.

Members of the prostaglanding family have been shown to be very potentphysiologically. Certain aspects of this work are described byBergstrom, Science 157, 382 (1967). Among the effects that have beenshown for prostaglandins are a marked lowering of blood pressure, eithercontraction or relaxation of smooth muscle tissue, and inhibition of therelease of free fatty acids from fat pads. For example, infusion of PGEin humans resulted in an increase in heart rate and a fall in arterialblood pressure.

Although the prostaglandins have been found in a num- VII United StatesPatent "ice her of tissues, their concentrations in these tissues areextremely small. For example, 13 different prostaglandins have beenfound in a total concentration of about 300 micrograms per milliter ofhuman seminal plasma. This is by far the highest concentration. observedin any tissue to date. Because of these extremely low concentrations itis impossible to obtain suflicient quantities of the prostaglandingsfrom natural sources. Among the more recent approaches to prostaglandinsynthesis are those reported by E. J. Corey and coworkers, J. Am. Chem.Soc. 90, 3245 and 3247 (1968).

SUMMARY We have now discovered a new synthetic route to members of theprostaglanding family from o-methoxyphenylacetic acid by introduction ofa proper acid side chain, conversion of the aromatic nucleus to thecyclopentane nucleus, and introduction of the second side chain. Oursynthesis is a multistep synthesis inolving a large number of chemicalreactions, all of which are known to those skilled in the art. Ourinvention lies in the application of these known chemical reactions tothe proper starting materials to arrive at the desired product.

In the course of our synthesis we have discovered a number of novelchemical compounds. Many of these compounds are useful as intermediatesin the synthesis of PGB, while others have been shown to have potentbiological activity in their own right. We claim these novel compoundsas a part of our invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The starting materail for oursynthesis is o-methoxyphenylacetic acid. This material is availablecommercially or may be readily synthesized from o-hydroxyphenyl aceticacid by formation of the methyl ether by any convenient means such as,for example, treatment with methyl sulfate. The series of reactionsinvolved in our synthesis may be depicted by the following schematicrepresentation.

The first step in the synthesis is the conversion ofomethoxyphenylacetic acid to the corresponding acid halide. Theconversion of a carboxylic acid to the corresponding acid halide is aWell-known reaction. Reagents that may be employed for this preparationinclude oxalyl chloride, phosphorus pentachloride, phosphorusoxychloride, phosphorus oxybromide, thionyl chloride, phosphorustribromide, and phosphorus triiodide. The reaction is preferablyconducted in an inert solvent such as chloroform, ether, methylenechloride, benzene, or toluene. The preferred acid halide for use in oursynthesis is the acid chloride.

Step B It is also Well known to react an acid halide with an enamine ofcyclopentanone to prepare a compound of the type depicted by FormulaIII. Enamines are obtained by the reaction of secondary amines withketones. We have found the morpholino enamine of cyclopentanone to beparticularly useful in our process. Other enamines that might beemployed include, for example, the pyrrolidino enamine and thepiperidino enamine of cyclopentanone. The reaction of the enamine withthe acid halide proceeds in the presence of a tertiary amine such astriethyl amine. This Well-known procedure is described, for example, inOrg. Syn. 41, 65. The reaction of the enamine with the acid halideactually results in an intermediate compound which must be furthertreated to obtain compound III.

Step C To obtain compound III from the intermediate obtained in Step Bit is necessary to treat the intermediate with a mineral acid. This stepis also a well-known step in the reaction of enamines with acid halides.

Step D The cyclopentaone ring of compound III is opened by treatmentwith a strong base. The product from this ring opening is the salt ofthe keto acid as depicted by Formula IV. This cleavage is the well-knownbase cleavage of 1,3- diketones as described in J. Am. Chem. Soc. 67,2204 (1945). The reaction is a well-known synthetic tool and is utilizedby us in the preparation of the acid side chain.

Step E is the addition of a mineral acid to the product I from Step D inorder to obtain the free, carboxylic acid depicted by Formula V from thesalt depicted by Formula IV, No further explanation of this reaction isnecessary. The ring opening and subsequent neutralization result in thecleavage of a small amount, about 10 percent, of the ether groups. Itis, therefore, advisable to remethylate the free hydroxyl groups at thispoint.

Step F Step G also involves reduction; however, in this step the benzenering is reduced to the cyclohexadiene ring. This reduction isaccomplished by means of the Birch reduction in which an alkali metaland ammonia are employed as the reducing agent. In our synthesis We havefound the use of lithium and ammonia in the presence of isopropylalcohol as solvent to be most advantageous. In this manner compoud VI isconverted to compound VII.

Step H Cleavage of the ether group in compound VII results in theformation of the ketone having Formula VII. We have found the use ofoxalic acid to be particularly Well suited for this cleavage reaction.

Step I Before proceeding further it is necessary to protect the carboxylgroup by the formation of an ester. The ester could have been preparedprior to the ether cleavage of Step H. Since the function of the esterpreparation is merely to protect the carboxyl group during subsequentreactions, the particular ester employed is unimportant. However, sinceit will be necessary to hydrolyze the ester in order to obtain PGB asthe free acid, it is suggested that theester formed be one that may beeasily cleaved. Thus R in Formula IX and subsequent formulas may bevirtually any group representing the alcohol portion of the ester. Forexample, R may be an alkyl or aralkyl group such as methyl, ethyl,t-butyl, 2,2,2-trichloroethyl, propyl, nonyl, benzyl, anddiphenylmethyl. From a practical standpoint, the ease of cleaving theester prepared must be balanced against the ease of preparation of theester. We have found the methyl ester to strike such a balance. Themethyl ester may be readily prepared by reacting the acid withdiazomethane. Other methods of ester preparation are well known to thoseskilled in the art. The particular method chosen does not affect oursynthesis.

Step I It is also necessary at this point to protect the carbonyl groupduring subsequent reactions. Methods of protecting carbonyl groups arealso well known to those skilled in the art. We have found that we canprepare a ketal from ethylene carbonate using p-toluene sulfonic acid ascatalyst in the presence of ethylene glycol. This ketal serves toprotect the carbonyl, function but may be readily removed by acidhydrolysis at a later step. Other carbonyl protecting groups may also beemployed.

Step K At this point in our synthesis we open the 6-membered ring. As afirst step in this ring cleavage the ozonide is formed at the doublebond by reacting compound X with ozone. Again, ozonolysis of a doublebond is a well known reaction. The addition of ozone to a double bondoccurs readily at low temperatures. Reductive cleavage of the ozonideresults in the formation of the dialdehyde XI. This may be accomplishedby treatment with zinc dust and acetic acid.

Step L The dialdehyde is then made to undergo ring closure to give acyclopentenal. This ring closure can occur, and in fact does occur, intwo ways so that two isomers, XII and XIII, are obtained. The ringclosure can be effected by treatment with a hindered secondary amine inan inert solvent. Preferred solvents are aromatic hydrocarbons such asbenzene, "toluene and xylene. The preferred hindered secondary amine is3-azabicyclo[3.2.2]-nonane. Other hindered secondary amines such as2,2,5,5-tetramethylpyrrolidine and 2,2,6,6-tetramethylpiperidine mayalso be used. The isomeric cyclopentenals formed are separated from eachother by chromatography using a mixture of chloroform and benzene assolvent on a silica column.

Step M The cyclopentenal to be employed in Step M is that rep resentedby Formula XII. In this step the long side chain is introduced into themolecule by the reaction of compound XII with the Wittig reagentobtained from triphenyl phosphine and l-bromoheptanone-Z. The Wittigreaction employed in this step of the synthesis is a well-knownreaction. The preparation of the Wittig reagent employed by us isdepicted by the following equations:

6 Step N Step 0 In order to regenerate the carbonyl group in the ringthe ketal is cleaved by treatment with acid. The acid cleavage of ketalsmay be effected with a strong organic acid such as p-toluene sulfonicacid or with a mineral acid such as hydrochloric or phosphoric acid. Itis this sensitivity to acid treatment that makes ketals so suitable forthe protection of carbonyl groups. We have found this particular ketalcleavage to proceed using p-toluene sulfonic acid in acetone at atemperature of 25 to 30 C.

Step P In order to convert compound XVI into an ester of PGB it isnecessary to shift the double bond in the ring. This isomerizationoccurs on treatment with a mild base. For example, treatment of XVI withdilute aqueous sodium hydroxide at room temperature results in completeisomerization to XVII in a few minutes. The isomerization will occurunder conditions sufficiently mild that no hydrolysis of the esteroccurs.

Step Q The esters of PGB, exhibit the same biological activity as doesthe free acid. Therefore, it is unnecessary to convert the ester XVII tothe free acid. However, for complete synthesis of PGB the ester may besubjected to basic hydrolysis to cleave the ester followed byneutralization with a mineral acid to yield PGB acid. Compound XVII isreadily hydrolyzed upon heating with aqueous alkali. Neutralization ofthe solution upon completion of the hydrolysis results in the formationof PGB Our invention will be further illustrated by the followingspecific examples:

Example 1. To a solution of 50 g. of o-hydroxyphenylacetic acid in 500ml. of benzene were added methyl sulfate and sodium hydroxide tomethylate the hydroxyl group. At the completion of the reactionapproximately ml. of the benzene was distilled in order to dry the acid.This solution was used directly in the next step.

Example 2.To the solution from Example 1 was added 96 ml. of oxalylchloride and the mixture was heated under reflux for 30 minutes. Thebenzene was removed by evaporation and the residue was distilled to give29.3 g. of acid chloride with a boiling point of 1100115 C. The acidchloride was used directly in the next step.

Example 3.The product from Example 2 was dissolved in 100 ml. of drychloroform and the solution was added over a 12-minute period to astirred solution of 22.6 g. of the morpholino enamine of cyclopentanoneand 14.97 g. of triethylamine in 100 ml. of dry chloroform. During theaddition the temperature of the mixture rose to 53 C. Upon completion ofthe addition the mixture was heated and stirred at 60 C. for one hour.To the mixture was then added 60 ml. of 6 N hydrochloric acid andstirring was continued at 55 C. for 30 minutes. The chloroform layer wasseparated and washed successively with water and saturated sodiumbicarbonate solution. This product gave a violet ferric chloride test inethanol. This solution of (o-methoxyphenyl)acetylcyclopentanone was useddirectly in the next step.

Example 4.-To the solution from Example 3 was added 6.2 g. of sodiumhydroxide in 180 ml. of 50 percent ethanol and the mixture was heatedunder reflux for 5 /2 hours. The solvents Were removed by evaporation.The residual oil was dissolved in water and the aqueous solution wasWashed twice with ether. This aqueous solution was then acidified to apH of 2 by the addition of concentrated hydrochloric acid. The acidifiedsolution was extracted with ether and the ether extract was washedsuccessively with 1 N hydrochloric acid, water, and saturated sodiumchloride solution, dried, and the ether evaporated to give 24.2 g. ofproduct. The crude product was recrystallized from a mixture of etherand petroleum ether to yield 19.5 g. of purified product having amelting point of 58-60 C.

During the course of this reaction a small amount, perhaps percent, ofthe methoxy groups are cleaved to the phenol so that it is recommendedat this point that the product be remethylated using methyl sulfate andsodium hydroxide as described in Example 1.

Example 5.-The keto acid from Example 4 was subjected to a Wolff-Kishnerreduction in accordance with the following procedure. A mixture of 19.5g. of the keto acid, 87 ml. of diethylene glycol, 32.9 ml. of 85 percenthydrazine hydrate, and 4.6 g. of potassium hydroxide was heated underreflux for three hours. To this mixture was then added 87 ml. ofdiethylene glycol and 23.9 g. of potassium hydroxide and the mixture wasagain heated to 200 C. The reaction mixture was cooled and diluted bythe addition of 600 ml. of ice and water. The mixture was acidified with60 m1. of concentrated hydrochloric acid and extracted with ether. Theether extract was evaporated to yield 18 g. of an oil which crystallizedon stand ing. This crystalline product had a melting point of 59 60 C.

Example 6.-To a three-neck, 12-liter flask equipped with a Dry Icecondenser and a mechanical stirrer were added 2 l. of ammonia that hadbeen distilled from sodium, 40.12 g. of the acid prepared as in Example5, and 1 l. of dry isopropanol. To this mixture was slowly added g. oflithium metal in pieces over a -minute period. When the solution turnedWhite another 10 g. of lithium was added and the mixture was refluxeduntil it turned white, approximately 25 minutes. To the mixture wasslowly added 320 ml. of ethanol followed by 500 ml. of water. Theammonia was removed by evaporation, and the mixture was acidified to apH of 1 by the addition of concentrated hydrochloric acid while cooling.The alcohol was stripped off and the residue was taken up in ether. Theether solution was washed successively with water and saturated sodiumchloride solution, dried, and evaporated to give the cyclohexadieneacid.

Example 7.At this point, the acid from Example 6 was treated withdiazomethane to yield 27.6 g. of the methyl ester. The esterificationreaction could as easily have been per-formed at some later step priorto the ozonolysis.

Example 8.The ester from Example 7 was converted to the cyclohexanone bydissolving in 700 ml. of ethanol and adding 85 ml. of water and 40 g. ofoxalic acid. The solution was allowed to stand at room temperature forthree hours. The ethanol was removed by evaporation and the residue wasextracted with ether. The ether extract was washed successively with 1 Nsodium hydroxide and saturated sodium chloride solution, dried, andevaporated to give 25.05 g. of product. The pure ketone was obtained bydistillation on a spinning band column with the product being collectedat an overhead temperature of 133 C. and a pressure of 1.5 mm. Thenuclear magnetic resonance, infrared, and ultraviolet spectra wereconsistent with structure IX.

Analysis.-Calculated for C H O (percent): 70.55; H, 9.31. Found(percent): C, 70.67; H, 9.48.

Example 9.-The ketone from Example -8 was converted to the ketal in thefollowing manner. In a 500 ml. flask were combined 8.0 g. of the ketoester from Example 8, 166 ml. of dried tetrahydrofuran, 43 ml. of driedethylene glycol, and 9.0 g. of ethylene carbonate. So this mixture wasadded 86 g. of p-toluene sulfonic acid and the mixture was allowed tostand at room temperature for five hours. At the end of this time thereaction mixture was dissolved in ether and the ether solution waswashed successively with 1 N sodium hydroxide, water, and saturatedsodium chloride solution, dried, and evaporated to yield 9.6 g. ofproduct. An analytical sample of the ketal was obtained by distillationon a spinning band column at 158 C. and 1.5 mm. pressure. The nuclearmagnetic resonance and infrared spectra were consistent withFormula X. I

Analysis-Calculated for C H O (percent): C, 68.05; H, 9.28. Found(percent): C, 68.40; H,'9.23.

Example 10.-A solution of 2.0 g. of the ketal ester from Example '9 in140 ml. of chloroform was cooled in a salt-ice bath and ozone was addeduntil the solution turned blue. This required approximately 30 minutes.The reaction mixture was flushed with nitrogen and transferred to a 500ml. suction flask cooled in an ice bath. The ozonide was decomposedunder nitrogen by the addition of 40 ml. of 75 percent acetic acid and4g. of zinc dust; This mixture was allowed to stand for one hour and wasfiltered. The filtrate was washed with water and the pH was adjusted to7 by the addition of saturated sodium bicarbonate solution. Thechloroform layer was then washed with water and sodium chloride solutionand dried over sodium sulfate. Evaporation of the chloroform gave 1.8 g.of an oil. The nuclear magnetic resonance and infrared spectra wereconsistent with the expected dialdehyde having structure XI.

Example 11.-A solution'of 10.3 g. of the dialdehyde prepared asdescribed in Example 10 in 100 ml. of benzene was added dropwise slowlyover a period of'one hour to a stirred, cooled (6 C.) solution of3-azabicyclo [3.2.2]nonane in 600 ml. of benzene. The reaction mix turewas stirred under a nitrogen atmosphere for an addi-' tional 45 minuteswith cooling to the extent that benzene crystals formed. The mixture wasslowly warmed to melt the benzene and then adjusted to pH 5 by. theaddition of 1 N hydrochloric acid. The benzene layer was separated,washed successively with water, sodium bicarbonate solution, andsaturated sodium chloride solution, and dried over sodium sulfate.Evaporation of the "benzene gave 8.54 g. of a brown oil which was shownto contain the isomeric cyclopentenals having structures XII and XIII aswellas some ketal ester having a fully saturated G-membered ring. Thisthree component mixture was separated, by chromatography on a silicacolumn using a weight ratio of product to silica of 1:10 and a solventconsisting of 20 percent chloroform and percent benzene. The columnemployed was 2 /2 cm. in diameter and 20 cm. long. Product XII wasidentified by nuclear magnetic resonance, in frared, and ultravioletspectra, and elemental analysis.

Analysis.-Calculated for C H O (percent): ,C, 64.84; H, 8.16. Found(percent): C, 65.06; H, 7.92.

Example 12.A Wittig reagent to be reacted with compound XII was preparedby the following sequence of steps.

(a) To 18 g. of red mercuric oxide in a 3-neck, 2-liter round-bottomflask, equipped with a mechanical stirrer and a 500 ml. addition funnelwere added 12 ml. of-an hydrous methanol and 12 ml. of borontrifluoride'etherate. The mixture became warm. It was heated withstirring to the boiling point of methanol and then cooled to roomtemperature. To this catalyst mixture was added 295 g. of chloroaceticacid. While the mixture was stirred and cooled 300 g. of l-heptyne wasadded dropwise over a one-hour period. The reaction mixture turned abrownblack color. The reaction was allowed to proceed at roomtemperature for an additional hour. The reaction mix-- ture wasextracted with 900 ml. of ether and the other layer was washed threetimes with water, three. times with sodium bicarbonate solution, andthree times with sodium chloride solution, and dried over sodiumsulfate. Evaporation of the ether left a brown oil which was distilledto give 342.8 g. of product boiling at 103 -107- C. at 9mm. pressure.

(b) To a stirred, cooled (-5 C.) solution of the 343 g. of the enolacetate. from (a) in 50 ml. of chloroform was added dropwise over athree-hour period a solution of 89 ml. of bromine in 50 ml. ofchloroform. The reaction mixture Was allowed to stand at roomtemperature for one hour. The chloroform was then stripped off undervacuum to give a brown solution which was distilled to give 179 g. ofproduct which boiled at 85 -:118 C. at 25 mm. pressure. This liquidproduct crystallized at 5 C.

(c) To a solution of 243 g. of triphenyl phosphine in 2 l. oftetrahydrofuran was added with stirring 179 g. of the bromoketone from(b). White crystals precipitated immediately. The reaction was stirredfor -8 days. The white crystals were removed by filtration and washedwith tetrahydrofuran to give 282 g. of product melting at 192-l93 C. TheWittig reagent was recovered from the salt by dissolving the salt inmethanol and adding potassium carbonate solution, This mixture wasstirred for approximately five minutes, the methanol was removed bystripping, and the mixture was extracted with ether. Evaporation of theether extract left the desired Wittig reagent. H H

Example l3.-To a solution of 0.141 g. of the cyclopentenal from Example11 in 10 ml. of 2B ethanol was added a solution of 0.178 g. of theWittig reagent from Example 12 in 10 ml. of 2B ethanol. This reactionmixture was heated under reflux under a nitrogen atmosphere for -18hours. The mixture was stripped to dryness and the residue waschromatographed on a silica column using a weight ratio of product tosilica of 1:20 and a solvent consisting of 20 percent chloroform and 80percent benzene. There was obtained a 43 percent yield of ketone havingstructure XIV. The nuclear magnetic resonance and ultraviolet spectrawere consistent with this structure and the molecular weight of 392 wasconfirmed by a high resolution mass spectrometer.

Analysis. Calculated for Cal-1 (percent): C, 70.37; 9.24. Found(percent): 0, 70.08; H, 9.45.

Example 14. A solution of 0.223 g. of the ketone from Example 13in ml.of dry tetrahydrofuran was added 'dropwise to a solution of 0.107 g. ofsodium borohydride in 10 ml. of dry tetrahydrofuran. The solution turnedyellow in color. The reaction mixture was heated under reflux in'anitrogen atmosphere for two hours and was then extracted with ether. Theether extract was washed with saturated sodium chloride solution untilit became clear. The ether solution was dried over sodium sulfate andevaporated to give 201 mg. of a yellow oil. This material waschromatographed on silica at a ratio of 1:33 using a solventwhich wasinitially percent chloroform and 80 percent benzene and finishing with100 percent chloroform. There'were obtained 47 mg. of a'fast-movingmaterial and 49 mg. of a slower-moving material. These two materialswere shown to be diasteroisomers of the desired alcohol. High resolutionmass spectrometry showed both to have a molecular weight of 394, whichis consistent with. structure XV, which has an empirical formula of C HO In addition, the nuclear magnetic resonance spectra of the. twoisomers were essentially identical as were the infrared and ultravioletspectra. Further, either isomer can be oxidized to the original ketoneby treatment with manganese dioxide.

Example 1S. T o a solution of 0.064 g. of the mixture of diasteroisomersfrom Example 14 in 30 ml. of acetone was added 0.0280 g. of p-toluenesulfonic acid. This mixture was then concentrated on a rotary evaporatorin an ice bath at 10 mm. pressure. The time required for evaporation wasapproximately seven minutes. To the residue was added 6.4 ml. of acetoneand the solution was spotted directly on a thin-layer chromatographyplate. The chromatogram indicated no reaction had taken place. Thevolume was made up to 30 ml. by the addition of acetone and the mixturewas again concentrated, this time at 27 C. To the residue was added 6ml. of acetone and -10 percent chloroform and percent benzene and slowlyincreasing chloroform to percent resulted in the isolation of 27 mg. ofa fast-moving material and 47 mg. of a slower-moving material. Thisslower-moving material absorbed at 233 nanometers and 278 nanometers.These two absorption bands were ascribed to the positions of the doublebonds in the isomeric compounds, XVI and XVII. Further purification bypreparative thin-layer chromatography gave a single-spot material whichwas shown by high resolution mass spectrometry to have a molecularweight of 350. This molecular weight is that expected for a compoundhaving the formula C H 4O Example l6.Isomerization of the ring doublebond of compound XVI to the position of compound XVII can be effected bytreating with sodium hydroxide. When the product from Example 15 wastreated with l N sodium hydroxide in ethanol there was a decrease in theabsorbance at 233 nanometers and an increase in the absorbance at 278nanometers. The absorbance at 278 nanometers has been shown for PGB andwould be expected for the methyl ester of PGB Example 17.-The methylester of PGB compound XVII, is converted to the free acid by heatingwith sodium hydroxide to saponify the ester followed by acidification torelease the free acid.

In Step L of our process two isomeric cyclopentenals depicted byFormulae XII and XIII are formed. Compound XII is used in the synthesisof PGB It is also possible to treat compound XIII in the same manner ascompound XII is treated in order to obtain an isomer of PGB The sequenceof reactions involved in the synthesis of this isomer is shown by thefollowing series of equations.

XX XIK turned yellowin eolor Thereaction mixture was evapo:

rated to dryness lan d the residue was chromatographed on silica at aratio of 1:40 :using a solvent consisting of 20 percent chloroform and80 percent benzene to give mg. of product. The nuclear magneticresonance, ultraviolet, and infrared spectra were consistent withstructure XVIII. High resolution mass spectrometry showed the product tohave a molecular weight of 392.26 which is also consistent withstructure XVIII.

Analysis.-Calculatcd for C H 0 (percent): 70.37; H, 9.25. Found(percent): C, 70.60; H, 9.38.

Example 19.-To a solution of 1.29 g. of the ketone from the precedingexample in 130 ml. of methanol was added, under nitrogen, 126 mg. ofsodium borohydride in 20 ml. of methanol. The solution turned yellow incolor. The reaction mixture was stirred at room temperature for twohours and then 25 ml. of water was added. The mixture was acidified to apH of 5 using 75 percent acetic acid while cooling the mixture to C. ina saltice bath. The mixture was extracted with ether and the etherextract was washed successively with sodium bicarbonate solution andsodium chloride solution, dried over sodium sulfate, and evaporated to1.07 g. of oil. T hin-layer chromatography showed only one majormaterial. The product was purified by chromatography on silica at a 1:20ratio employing 30 percent chloroform, 70 percent benzene as solvent.The nuclear magnetic resonance, infrared, and ultraviolet spectra areconsistent with the expected structure XIX. High resolution massspectrometry showed the product to have a molecular weight of 394.27 ascompared to the calculated molecular weight of 394.53.

I Analysis.-Calculated for C H O (percent): 70.01; H, 9.71. Found(percent): C, 69.68; H, 9.62.

Example 20.To a solution of 0.718 g. of the ketal from the precedingexample in 150 ml. of acetone was added 0.066 g. of p-toluene sulfonicacid with stirring. A yellow color developed. The reaction was allowedto proceed for two hours under nitrogen with stirring. The reactionmixture was extracted with ether and the ether extract was washedsuccessively with sodium bicarbonate and sodium chloride solutions,dried over sodium sulfate, and evaporated to 638 mg. of a dark oil. Theproduct was purified by chromatography on silica at a 1:25 ratio,starting with a solvent consisting of percent chloroform and 80 percentbenzene and slowly increasing the chloroform content to 100 percentchloroform. The nuclear magnetic resonance, infrared, and ultravioletspectra are consistent with the expected structure XX. The molecularweight by high resolution mass spectrometry was 350.25 which isconsistent with the formula C H O Analysis.-Calculated for C H O(percent): 71.96; H, 9.78. Found (percent): C, 71.50; H, 10.04.

Example 21.-To obtain the free acid from the ester from the precedingexample, 102 mg. of the ester was dissolved in 15 ml. of methanol. Tothe solution was added, under nitrogen, 125 mg. of sodium carbonate in15 ml. of water. This reaction mixture was heated for one hour at 60 C.then cooled in ice to 10 C. and acidified to a pH of 2 using 50 percenthydrochloric acid. The reaction mixture was extracted with ether and theether extract washed with water, made basic with l N sodium hydroxide,and reacidified, while cooling, with 1 N hydrochloric acid. The etherextract was then washed with water and saturated sodium chloridesolution, dried over sodium sulfate, and evaporated to give 84.6 mg. ofthe desired acid.

It is also possible to subject various intermediates in our process toother reactions to obtain compounds that are structurally similar to theprostaglandins. For example, the ketal/ketone XIV may be subjected toacid hydrolysis to remove the ketal blocking group and yield a diketone.This reaction is depicted by the following equation.

o emu-im m; o

XIV 1 XXI The conversion of XIV to XXI involves the acid hydrolysis of aketal to yield the ketone. This is the same as Step 0 in the mainprocess. The hydrolysis may be accomplished by the use of a strongsulfonic acid or a strong mineral acid. For example, p-toluene sulfonicacid,

12 hydrochloric acid, or sulfuric acid may be used. This hydrolysis willbe further illustrated by the following example.

Example 22.To a solution of 0.142 g. of the ketal XIV in 50 ml. ofacetone was added 0.142 g. of p-toluene sulfonic acid. The system wasflushed several times with nitrogen and the reaction mixture was heatedunderreflux in a nitrogen atmosphere for 24 hours. The mixture wasextracted with ether and the ether extract washed with saturated sodiumchloride solution, dried over sodium sulfate, and evaporated to give 155mg. of yellow oil. The yellow oil was chromatographed on silica at aratio of 1:32 using percent chloroform, percent ben zene as solvent togive 52 mg. of a product which showed to be single-spot material bythin-layer chromatography. The infrared, ultraviolet, and nuclearmagnetic resonance spectra of the product were consistent with theexpected structure XXI. In addition, the molecular weight as determinedby high resolution mass spectrometry was the expected 348.

AnaZysis.Calculated for C H O (percent): C, 72.38; H, 9.26; O, 18.37.Found (percent): C, 71.70;H, 9.30; O, 18.12.

It is to be understood that compound XXI can be hydrolyzed to the freeacid. Such hydrolysis is well within the skill of a trained chemist. Itis also to be understood that compound XX-I may be prepared in otherways and that other compounds may be obtained from the intermediates ofour process. For example, compound XXI may be obtained by the oxidationof the hydroxyl group in the side chain of compound XVI. This oxidationmay be accomplished by the use of a mild oxidizing agent such asmanganese dioxide.

The diketone XXI is reduced on treating with lithium aluminumtri-t-butoxy hydride to the hydroxy ketone XXI I.

XXI

o l IIFCHiIC H Compound XXII is a new synthetic prostaglandin whichcauses a decrease in blood pressure upon administration. The conversionof XXI to XXII is illustrated by the following example. Example 23.Asolution of 0.182 g. of the diketone XXI in 20 ml. of tetrahydrofuranfreshly distilled from lithium aluminum hydride was cooled to -74 C. ina Dry Ice/isopropanol bath. To this cooled solution was added a solutionof 0.140 g. of lithium aluminum tri-tbutoxy hydride in 20 ml. of freshlydistilled tetrahydrofuran over a 13-minute period, with stirring andunder nitrogen. Stirring was continued under nitrogen at -74 C. for onehour. The mixture was then warmed to room temperature and held there for30 minutes. The product Was extracted from the mixture with ether, theetherextract was washed with saturated sodium chloride solution, driedover sodium sulfate, and evaporated to give 158 mg. of a brown oil. Theproduct .was'purified' by chromatography over silica using a 20 percentchloroform/ 80 percent benzene solvent and a product to silica weightratio of 1:11. The infrared and ultraviolet spectra were consistent withstructure XXII and the molecular weight of 350 was confirmed by massspectroscopy.

In the course of our work we have discovered a method for thepreparation of the naturally occurring prostaglandin, PGB and inaddition have prepared two heretofore unknown synthetic prostaglandins,compounds XVI and XXII. These two new prostaglandins have the formulae:

on cii=cncnc it 13 It is to be understood that these compounds may existas free acids, or the methyl group may be replaced by such groups asethyl, propyl, t-butyl, nonyl, 2,2,2-trichloroethyl, benzyl, anddiphenylmethyl.

These new prosta'glandins exhibit many of the desirable characteristicsof the prostaglandin family but surprisingly are free from many of theundesirable effects. For example, both these compounds cause a markeddecrease in blood pressure, which is characteristic of theprostaglandins; however, neither compound causes contraction orrelaxation of smooth muscle tissue, which is another characteristic ofthe naturally occurring prostaglandins.

The effect of compounds XVI and XXII on the blood pressure of ananesthetized rabbit was determined according to standard procedures. Theanimals employed were male NewZealand White rabbits anesthetized withurethane (1.25 g./1 kg.) or Nembutal (25 mg./1 kg). The animal wasprepared for recording of blood pressure using the Statham pressuretransducer. The compound was dissolved in ethanol-water and the solutionwas administered via the femoral vein. Results are given as follows: (a)mean blood pressure just prior to administration of the compound; (b)mean blood pressure after administration; and (c) the change in bloodpressure. The results are summarized in the following table.

Dosage,

ig/kg. Results Compound 1 Animal expired.

@Jcng coza O -OCH 4 VI VIII 0 be I CF (Ciizhco clt O l l CH XI XII l(crral co cn In the above formulae it is to be understood that themethyl group may be replaced by hydrogen or an alkyl or aralkyl group.The methyl group may be replaced by such groups as ethyl, propyl,t-butyl, nonyl, 2,2,2-trichloroethyl, benzyl, and diphenylmethyl. Suchcompounds are equivalent to the methyl esters we have shown. One skilledin the art can readily go from the acid to an ester or from the esterback to the acid.

A skilledchemist will recognize that our process can be modified invarious aspects Without changing the essence of the invention. Forexample, the carboxyl group of the intermediate compounds may beconverted to an ester at some" point in the synthesis other than Step I.Too, it is possible to use other ketone blocking groups than the one wehave used. The essence of our invention lies in the application of thevarious steps we have described, although not necessarily in the orderwe have described, to the proper starting material.

We claim: j

1. A method for the synthesis of a prostaglandin which comprises thesteps of:

(A) converting (o-methoxyphenyl)acetic acid to the corresponding(o-methoxyphenyl)acetyl halide having the formula II CHzCX OCH 0 0 I llonto o on III (D) subjecting III to basic hydrolysis to yield a salt of7-(o-methoxyphenyl)-6-oxoheptanoic acid having the formula wherein M isthe cation derived from the base employed in the hydrolysis;

(E) treating IV with a mineral acid to liberate the free carboxylic acid(V) (F) subjecting V to a Wolff-Kishner reduction to obtain7-(o-methoxyphenyl)heptanoic' acid (VI) OCH 15 (G) subjecting VI to aBirch reduction to convert the benzene ring to a cyclohexadiene ring,yielding an acid having the formula OCH3 VII

(H) cleaving the ether group of VI-I to yield a ketone having theformula VIII (I) converting VIII to an ester having the formula (CHzhC02R wherein R is an alkyl or aralkyl group; (J) treating IX withethylene carbonate in the presence of a strong acid catalyst to form theketal (X) to protect the carbonyl group during subsequent reactions (0H2) 0 C 0 2R 1 o (K) treating X with ozone, followed by reductivecleavage of the intermediate ozonide to the dialdehyde having theformula (L) treating XI with a hindered secondary amine selected fromthe class consisting of 3-azabicyclo- [3 2- 2]nonane,2,2,5,S-tetramethylpyrrolidine, and 2,2,6,6-tetramethylpiperidine toeffect ring closure to a mixture of isomeric cyciopentenals having theformulae )OHQsCO R (CHzhCO R h l 0 HO o 0 5H: A] EH3 on, cfig XII XIII16.. (M) subjecting X-lI to a. Wittig reaction by treatment with areagent having the formula to yield a ketone having the formula I 0 el gon-(imam XIV r (N) reducing the ketone group of XIV to yield an alcoholhaving the formula I i v V 011 v i. ywrrmooga (L -0 r o r XVI (P)treating XVI with a mild base .to effect isomerization to PGB ester,having the formula XVII I a (Q) subjecting XVII to basis hydrolysis tocleave the ester, followed by neutralization with a mineral acid toyield PGB acid.

References Cited Morin et a1.: Tet. letters, 1968, 6023. Daniels et al.JAOS 5894, 1968.

.9 LORRAINE A. WEINBERGER, Primary Examiner R. GERSTL, AssistantExaminer US. Cl. X.R.

c5 zed-346.9, 396, 468 R, 521 R, 590

